KEYPORT — When Brian Allen walked this beach 10 years ago, it was a bare mud flat that sucked and held his boots in an oozing grip.

Now he treads easily, his feet supported by millions of rare Olympia oysters.

"There are not many places like this — at all," said Allen, a marine ecologist with Bainbridge Island-based Puget Sound Restoration Fund. "But we built it, and they came."

Pushed to the brink of extinction by pollution and overharvesting, Puget Sound's native oyster has been the focus of several revival efforts, but this 10-acre restoration area on Dogfish Bay, about a mile south of Poulsbo, is the first to foster a self-sustaining and growing population.

About the size of a 50-cent piece, the Olympia is smaller and slower growing than the Pacific oyster, a foreign import that now dominates the sound and is the oyster of choice at supermarkets and restaurants.

"Pacifics have more meat and get bigger quicker," Allen said. "So, from a commercial point of view, they're a better product."

But from an ecological perspective, Olympias are priceless. They form three-dimensional habitat supporting a host of other species, from Dungeness crab to juvenile salmon.

"They're ecosystem engineers that form habitat for other species," said Eric Buhle, a National Oceanic and Atmospheric Administration ecologist who helped with the PSRF-led restoration. "Olympia oysters are a foundation species."

That was evident on a single clump of Olympias Allen pulled from the beach.

"Look at all of them," he said, pointing to limpets, chitons and other mollusks living on the oysters. "It used to be that all the life here was in the mud. Now its above as well, with crab, shrimp, fish, birds. They're here now because this is where all these little beasties are."

Olympias began their decline during the California Gold Rush. The oysters were dredged by the ton and sent down to feed hungry '49ers.

"They have a more complex flavor than you get from most Pacific oysters," Allen said, describing a raw Olympia's "briny, earthy" start and "clean, almost metallic" finish. Oyster eaters thinking about snagging Olympias from Dogfish should take heed of the most recent shellfish harvest closure, which was announced Wednesday, and other recent closures rooted in pollution from human waste. The high levels of fecal bacteria don't hurt shellfish but can make them poisonous to humans.

Gold Rush-era demand for oysters depleted Puget Sound of oysters and made it difficult for them to launch a comeback. Young oysters need old oyster shells to latch onto and begin the final stages of development. With tons of shells dumped into San Francisco Bay, Olympias began to fade from the sound. Pollution from mills and other industrial sources sped their decline.

Pacific oysters were brought from Japan as a quick-growing replacement. Evolved for life on another continent, Pacifics prefer warmer temperatures and lower acidification than the waters around Washington state. They do well in the sound only under certain conditions and at certain depths, whereas the Olympia can thrive almost anywhere, so long as there's enough shell to foster subsequent generations.

Once covering up to 20,000 acres in the sound, Olympias have dwindled to just 5 percent of their former abundance, according to a study by the state Department of Fish and Wildlife.

PSRF hopes to revive at least 100 acres by 2020. The small nonprofit group is focusing its efforts on 19 areas identified by Fish and Wildlife as prime Olympia habitat. To boost supply, PSRF established an Olympia nursery at the federally managed Manchester Research Station in South Kitsap. The nursery produces millions of gravel-sized oyster babies for "seeding" in targeted areas.

The Dogfish Bay restoration began in 2005. The bay had a lot going for it. Tucked into a quiet corner of Liberty Bay, Dogfish has a slow rate of water circulation that aids in oyster reproduction and larval growth. It takes about a month for water to flush out of the bay, giving free-floating larvae time to find a suitable spot to grow.

Borrowing a barge and a loading pier from the nearby Keyport Navy facility, PSRF began spreading dump truck loads of Pacific oyster shell acquired from commercial oyster operations. Olympia shells were preferred, but large quantities are hard to come by. PSRF added no oyster seed, hoping that a tiny patch of Olympias already growing in the bay would colonize the shells.

PSRF continued to add shell but not much happened until 2010.

"That's when we hit the tipping point," Buhle said.

"We went from about a dozen oysters per meter to about a hundred oysters per meter, and it keeps growing," Allen added.

An increasingly dense population of Olympias now cover 10 acres.

Unlike other restoration sites run by PSRF and other groups like Mount Vernon-based Northwest Straits Commission, the Dogfish project no longer needs shell infusions.

It's robust enough to not only produce its own shell but send larvae out to colonize other areas, potentially sparking revivals in Liberty and the bays on Bainbridge's west side.

Allen hopes to replicate the success in Dogfish at other sites around the sound.

"This is the first place that's actually turning into the objective," Allen said. "We want to be able to give a kick to get things moving over the threshold so the natural process takes over. It's very cool that now we know we're on the right path."